Continuous-casting mold and a process for the continuous casting of thin slabs of metal

ABSTRACT

The invention relates to a process and a continuous-casting mold for casting thin slabs. The mold has an oblong inner cross-sectional area and cooled mold walls. The melt is poured in through at least one delivery nozzle which dips into the melt. To ensure that, during casting, markedly lower stresses and, as a consequence thereof, fewer cracks appear in the strand shell, at least at the casting level being established and at least over a part of the depth of immersion of the delivery nozzle, the ratio of the gap widths S TI  and S II/2  and the ratio of the cooling capacities L TI  and L II  of the mold wall are related by the equation: 
     
         [S.sub.TI /(S.sub.II /2)]/[L.sub.TI /L.sub.II ]&gt;1. 
    
     S TI  is the width of the gap formed in the zone immediately surrounding the particular immersed delivery nozzle by the outer surface of the delivery nozzle and by the inner surface of the directly opposite mold wall, and S II/2  is half the width of the gap formed by the inner surfaces in the zones in which the inner surfaces of the mold walls are directly opposite each other. L TI  and L II  are the cooling capacities of the zones of the mold wall which form the respective gap or gap section.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a continuous-casting mold for casting thinslabs, the mold having an oblong inner cross-sectional area and cooledmold walls. The melt is poured in through at least one delivery nozzlewhich dips into the melt. The invention further relates to a process forcontinuously casting thin slabs.

2. Discussion of the Prior Art

In the continuous casting of strands having an oblong cross section, itis known to form the inner cross-sectional area of thecontinuous-casting mold so that a strand section as close as possible tothe desired final dimensions is produced by the continuous-casting mold.In this case, especially those section beams having an H-shaped crosssection and also those having a cross section in which thecross-sectional ends have thickenings (dog bone-shaped cross section),the problem regularly arises that the ends, which are widened and/orthickened relative to the web width, of the section beam frequently showcracks and stresses and/or undesired crystal structures are cast closeto the final dimensions. In the case of section strands not cast closeto the final dimensions, however, technically involved andcost-intensive rolling processes are required after casting in order toobtain the desired final dimensions.

DE 2,034,762 A1 has disclosed a process and apparatus for producing athin strip, in which the strip has a thickening which extends in itslongitudinal direction and which still has a liquid core. Thisthickening is then forced back underneath the mold by pressure rollers.

U.S. Pat. No. 5,082,746 discloses specially dimensioned section strandswhich must not exceed predetermined cross-sectional parameters aridwhich have a predetermined homogeneous crystal structure, so that thedesired cross-sectional profile can then be obtained with the minimum ofrolling work. Such section strands can, as experience shows, be castusing one or more delivery nozzles for pouring in the melt. In thiscase, it has been found that merely the restriction of thecross-sectional parameters and the setting of a desired crystalstructure are not sufficient to produce section strands close to thefinal dimensions without cracks, and with a homogeneous crystalstructure over the entire cross section. It is also insufficient, in thecase of a strand section with flanks molded onto the ends, to select theweb width to be equal to the flank width, as is explicitly suggested inU.S. Pat. No. 5,082,746. In fact, section strands produced specificallyunder these conditions regularly show cracks and, particularly in thezone of the flanks, a less favorable crystal structure than that of theweb, which indicates that uniform casting conditions in eachcross-sectional zone during casting with the use of immersed deliverynozzles cannot simply be achieved by adhering to limiting values of theabove-mentioned cross-sectional parameters.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a process and acontinuous-casting mold having cooled mold walls for casting strandshaving an oblong inner cross-sectional area, for example section strandshaving an H-shaped cross section and a predetermined web width, the meltbeing poured in by at least one delivery nozzle which dips into themelt, in which mold markedly lower stresses arise during casting and, asa consequence thereof, fewer cracks appear in the strand shell.Furthermore, the cast strands should have a homogeneous crystalstructure over the entire cross section.

The invention provides that, at least at the casting level beingestablished at least over a part of the depth of immersion of thedelivery nozzle, the ratio of the gap widths S_(TI), in the zoneimmediately surrounding the delivery nozzle, and S_(II) /2, in the zonesin which the inner surfaces of the mold walls are directly opposite oneanother, and the ratio of the cooling capacities L_(TI) and L_(II) ofthe corresponding zones of the mold wall (1, 2) are related by theequation:

    [S.sub.TI /S.sub.II /2)]/[L.sub.TI /L.sub.II ]>1.

S_(TI) here is the width of the gap formed by the outer surface of theparticular delivery nozzle and by the inner surface of the directlyopposite mold wall. S_(II) /2 is half the width of the gap formed by theinner surfaces and, in particular in the zones in which the innersurfaces of the mold walls are directly opposite each another, i.e. inwhich no delivery nozzle is located between the inner surfaces. L_(TI)and L_(II) are the cooling capacities of the mold wall in thecorresponding zones.

The continuous-casting mold having an internal cross section dimensionedin this way makes it possible to uniformly melt casting flux resting onthe casting level even at high casting speeds and to take it offuniformly together with the slag. This leads to the formation of amolten slag/casting flux layer of uniform height over the entire innercross-sectional area. A slag/casting flux layer of uniform heightadvantageously effects, during continuous casting, the formation of auniform slag/casting flux layer between the mold wall and the strandsurface. In this way, very good sliding of the strand shell along theentire mold wall can be ensured and the heat of the melt or of thestrand can be removed very uniformly through the mold walls duringcasting, so that a strand shell having a very homogeneous crystalstructure and no stresses and cracks is formed.

Advantageously, [S_(TI) /(S_(II) /2)]/[L_(TI) /L_(II) ] is between 1.05and 1.30 over the entire depth of immersion of the delivery nozzle and,hereby in particular the influence of the wall of the delivery nozzleupon the thermal conditions in the mold during casting is taken intoaccount.

With the uniform cooling of the mold walls, the dimensioning of therequired internal cross section of the continuous-casting mold can besimplified so that [S_(TI) /(S_(II) /2)]>1 applies, and preferably[S_(TI) /(S_(II) /2)] is between 1.05 and 1.30, whereby, in particular,the influence of the wall of the delivery nozzle upon the thermalconditions in the mold during casting is again taken into account.

If the delivery nozzle is located in the web zone, pursuant to theinvention the delivery nozzle has an oblong cross section. As a result,the zones of the long sides opposite the delivery nozzle have to beshaped outward only to a relatively small extent.

The invention also proposes, in particular for producing a cross sectionhaving thickened ends (dog bone shaped), to locate two delivery nozzlesin the zone of each of the short sides. In this case, it is ofadvantage, with regard to the final dimensions, if the delivery nozzlesthen have, for example, a substantially triangular cross section.

For cooling the mold walls, cooling elements are used, for examplecooling tubes, which are distributed over the mold walls per unit areain such a way that the cooling capacity intended in the correspondingzone is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

An illustrative embodiment of the invention is shown in the drawing andis described in more detail below, where:

FIG. 1 shows a cross section of a continuous-casting mold when operatedwith a central delivery nozzle, and

FIG. 2 shows a cross section of a continuous-casting mold when operatedwith two delivery nozzles arranged on the short sides and each having atriangular cross section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a cross section through a continuous-casting mold having anoblong inner cross-sectional area at the casting level established forcasting strands. The long-side mold walls 1, 1 and the short-side moldwalls 2, 2 are each arranged mutually opposite to form a castingchamber. The walls 1, 2 preferably consist of copper and are providedwith cooling tubes 3 for removing heat. The cooling tubes 3 here ensureuniform heat removal via the mold walls 1, 2, since an appropriatenumber of cooling tubes 3 in the mold wall 1, 2 is provided per unitarea. With the mold shown in FIG. 1 in operation, a delivery nozzle 4,which dips into the melt and preferably has an oblong cross section, iscentrally arranged for pouring in the melt.

FIG. 1 shows that, in the immediate surroundings of the delivery nozzle4, the long-side mold walls 1, 1 are each curved outward, namely in sucha way that the gap 7, formed between the long-side mold walls 1, 1 andthe delivery nozzle 4, has a substantially constant gap width STI overthe entire depth of immersion of the nozzle 4. This is achieved in theillustrated embodiment shown in FIG. 1 in such a way that the outersurfaces 6 of the delivery nozzle 4 have a contour similar to that ofthe immediately opposite inner surfaces 5 of the long-side mold walls 1.Due to the oblong shape of the delivery nozzle 4, the zones of the longsides 1 opposite the delivery nozzle 4 have to be outwardly shaped to arelatively small extent.

In the remaining zones to the left and to the right of the deliverynozzle 4, the directly opposite inner surfaces 8 of the long-side moldwalls 1, i.e. without the delivery nozzle located in between, form a gap9, one half of whose gap width S_(II) /2 is at most equal to S_(TI),i.e. the gap width of the directly opposite inner surfaces 8 is at mosttwice the gap width S_(TI) of the gap 7.

A further embodiment of a continuous-casting mold having an innercross-sectional area dimensioned according to the invention is shown inFIG. 2. The continuous-casting mold shown in FIG. 2 has, in the zone ofthe short-side mold walls 2, an enlargement of the mold interior, ineach of which a delivery nozzle 4 is located (cross section withthickened ends, also known as dog bone cross section). The outer crosssection of the delivery nozzle 4 can be of almost any desired shape; inthe illustrative embodiment according to FIG. 2, the delivery nozzle 4is of substantially triangular outer cross section. In the zone of thedelivery nozzle 4, the gap 7 formed by the outer surface 6 of thedelivery nozzle 4 and the directly opposite inner surface 5 of the moldwall is again dimensioned over the entire depth of immersion so that thegap width S_(TI) is substantially constant.

In the middle zone of the continuous-casting mold, where the innersurfaces 8 of the mold long-side walls 1 are directly opposite, formingthe gap 9, half the width S_(II) /2 of the gap 9 is somewhat less thanS_(TI) ; the gap 9 itself is thus again at most twice the width S_(TI)of the gap 7 in the zone of the section ends.

A substantially constant gap width in the illustrated embodiments meansthat, in relatively small zones, i.e. for example in the corners of thetriangular cross section of the delivery nozzle 4, variations from thedemanded uniformity of the gap width can arise. Consequently, theuniformity of the gap width must only be approximately met in thesezones, but it should not exceed twice the value. In the same way, theflanks--as can be seen in the left-hand half of FIG. 1--can be shapedsomewhat outward. of course, the gap width in both illustrativeembodiments can be reduced or enlarged if, in the zone of the gap 7, thecooling capacity of the mold long-side wall 1 is, respectively, smallercr greater in the corresponding zones. The decisive point is that theratio of gap width (S_(TI) or S_(II) /2) and cooling capacity (L_(TI)and L_(II) respectively) of the corresponding zone of the mold wall 1 isconstant at each point of the continuous-casting mold and is preferablywithin the range between 1.05 and 1.30. In the illustrative embodiments,this value is 1.05.

During operation of the continuous-casting mold according to FIG. 1 orFIG. 2, the mold is continuously filled with molten steel via thedelivery nozzle or nozzles 4, and the cast section strand is taken offat constant speed. During the casting with constant take-off speed,exactly the same quantity of molten steel is continuously poured in asthat taken off at the mold outlet, so that the casting level beingestablished is constant with continuous renewal of the molten steelremaining in this zone, and this additionally effects the melting of thecasting flux introduced and lying on the casting level. The essentiallyconstant gap width in the illustrative embodiments according to FIG. 1and FIG. 2 then ensures a uniform upward-directed heat flux in allcross-sectional zones of the continuous-casting mold, so that, in thezone of the casting level, uniform melting of the casting flux takesplace, i.e. the same quantity of casting flux is always melted per unitsurface area of the casting level in per unit time. In addition, at aconstant take-off speed of the cast section strand, the slag/castingflux layer being formed establishes itself at the same height at eachpoint of the inner cross-sectional area in the casting level zone as aresult of the inner cross-sectional shape according to the invention.Connected thereto is a slag/casting flux film, likewise beingautomatically established, of constant thickness between the mold wall1,2 and the melt or strand shell at all points of the strand surface.

Due to the specific dimensioning of the mold and the slag/casting fluxfilm of constant thickness, thereby being established during casting, aquantity of heat proportional to the wall area is continuously removedfrom the molten steel in the zone of the mold walls and the melt isuniformly cooled to form the strand shell. The quantitative influence ofthe slag/casting flux film results directly from the specific thermalconductivity thereof and the thickness of the film being established. Aconstant thickness of the mold wall 1,2 effects, at a given temperaturedifference, a constant thermal resistance during the removal of thequantity of heat from the melt through the mold walls 1,2. The totalthermal resistance results from the sum of the individual partialthermal resistances, into which the reciprocals of each of the specificthermal conductivities of the layers (mold wall--slag/castingflux--strand shell melt--wall of the delivery nozzle) located one behindthe other enter. The specific thermal conductivity of the slag/castingflux film is about 1 W/Km and is thus determining for the heat removaland hence for the cooling of the strand, as has been shown byexperimental investigations. By means of the invention, the heattransition into the mold is made uniform over the entire mold length inthe horizontal direction via the constant thickness of the slag/castingflux film being established. Temperature differences in the strandshell/mold wall boundary zone are greatly reduced in this way, so thatonly slight stresses are then still present in the strand shell of thecast strand, which greatly reduces the danger of cracks forming. Inaddition, as a result of the very good uniform lubrication thusobtained, the walls of the continuous-casting mold are exposed toreduced wear, so that additionally their service life is markedlyextended.

I claim:
 1. A continuous-casting mold for casting thin slabs,comprising:cooled mold walls that define an oblong inner cross-sectionalarea; and a delivery nozzle which pours melt into the mold and whichdips into the melt, the mold walls being configured so that, at least ata casting level established at least over a part of a depth of immersionof the delivery nozzle into the melt, a ratio of gap widths (S_(TI) andS_(II) /2) and a ratio of cooling capacities (L_(TI) and L_(II)) of themold wall are related by the equation:

    [S.sub.TI /(S.sub.II /2)]/[L.sub.TI /L.sub.II ]>1,

where S_(TI) is the width of a gap formed in a zone immediatelysurrounding the delivery nozzle by an outer surface of the deliverynozzle and by an inner surface of the mold wall, and S_(II) /2 is half awidth of a gap formed by inner surfaces of the mold walls in zones inwhich the inner surface of the mold walls are directly opposite eachother, and L_(TI) and L_(II) are the cooling capacities of the zones ofthe mold wall which form the respective gaps.
 2. A continuous-castingmold as defined in claim 1, wherein the ratio of the gap widths S_(TI)and S_(II) /2 and the ratio of the cooling capacities L_(TI) and L_(II)of the corresponding zones of the mold wall are related by the equation:

    [S.sub.TI /(S.sub.II /2)]/[L.sub.TI /L.sub.II ]=1.05-1.30.


3. A continuous-casting mold as defined in claim 1, wherein the moldwalls are configured to have a uniform cooling capacity, the ratio ofthe gap widths S_(TI) and S_(II) /2 being

    [S.sub.TI /(S.sub.II /2)]>1.


4. A continuous-casting mold as defined in claim 1, wherein the moldwalls are configured to have a uniform cooling capacity, the ratio ofthe gap widths S_(TI) and S_(II) /2 being

    [S.sub.TI /(S.sub.II /2)]=1.05-1.30.


5. A continuous-casting mold as defined in claim 1, wherein the deliverynozzle has an oblong cross section.
 6. A continuous-casting mold asdefined in claim 1, wherein the delivery nozzle has a substantiallytriangular cross section.
 7. A continuous-casting mold as defined inclaim 6, wherein the mold walls include short side walls and long sidlewalls that extend between the short side walls, a separate deliverynozzle being located in a region of each of the short side walls.
 8. Acontinuous-casting mold as defined in claim 1, and further comprisingcooling elements arranged at the mold walls so as to have a distributionthat matches a desired cooling capacity.
 9. A process for continuouscasting of thin slabs having an oblong inner cross-sectional area,comprising the steps of:providing a mold having cooled walls; andpouring melt into the mold via at least one delivery nozzle that dipsinto the melt, wherein, at least at a casting level being established atleast over a part of a depth of immersion of the delivery nozzle intothe melt, a ratio of gap widths (S_(TI) and S_(II) /2) and a ratio ofcooling capacities (L_(TI) and L_(II)) of the mold walls are related bythe equation:

    [S.sub.TI /(S.sub.II /2)]/[L.sub.TI /L.sub.II ]>1,

where S_(TI) is the width of a gap formed in a zone immediatelysurrounding the delivery nozzle by an outer surface of the deliverynozzle and by an inner surface of the mold wall, and S_(II) /2 is half awidth of a gap formed by the inner surfaces of the mold walls in zonesin which the inner surfaces of the mold walls are directly opposite eachother, and L_(TI) and L_(II) are the cooling capacities of the zones ofthe mold walls which form the respective gaps.
 10. A process as definedin claim 9, wherein, for the entire depth of immersion of the deliverynozzle, the ratio of the gap widths S_(TI) and S_(II) /2 and the ratioof the cooling capacities L_(TI) and L_(II) of the corresponding zonesof the mold walls are related by the equation:

    [S.sub.TI /(S.sub.II /2)]/[L.sub.TI /L.sub.II ]=1.05-1.30.


11. A process as defined in claim 9, wherein the mold walls have auniform cooling capacity and the ratio of the gap widths S_(TI) andS_(II) /2 is

    [S.sub.TI /(S.sub.II /2)]>1.


12. A process as defined in claim 9, wherein the mold walls have uniformcooling capacity, the ratio of the gap widths S_(TI) and S_(II) /2 is

    [S.sub.TI /(S.sub.II /2)]=1.05-1.30.


13. A process as defined in claim 9, wherein the delivery nozzle has anoblong cross section.
 14. A process as defined in claim 9, wherein thedelivery nozzle has a substantially triangular cross section.
 15. Aprocess as defined in claim 14, wherein the mold walls include shortside walls and long side walls that extend between the short side walls,the pouring step including pouring melt into the mold with a separatedelivery nozzle located in a region of each of the short side walls. 16.A process as defined in claim 9, including cooling the mold walls withcooling elements having a distribution that matches a desired coolingcapacity.